From Combustion By-Product to Geotechnical Cement–Soil Composite: Computed Tomography-Informed Numerical Modelling and Experimental Validation

The partial replacement of cement with waste-derived materials is a promising strategy for reducing cement consumption in soil–cement technologies and developing more sustainable construction materials. This study investigates fluidized bed bottom ash (BA), an industrial combustion by-product, as a partial Portland cement substitute in geotechnical cement–soil composites. Cement was replaced with BA at 5%, 10%, and 15% by mass, and specimens were tested after 3, 7, 14, and 28 days of curing. The research program combined SEM-EDS characterization, high-resolution X-ray computed tomography, uniaxial compression tests, in situ CT observations during loading, and FLAC3D mesoscale numerical modelling calibrated against experimental stress–strain curves and observed crack propagation. The results showed a non-monotonic effect of BA addition. Up to 10% BA, the median pore size remained relatively stable, whereas 15% BA increased porosity and enlarged the upper tail of the pore-size distribution. The calibrated model reproduced the global mechanical response with acceptable accuracy and captured main features of damage localization. The analyses are accompanied by a life cycle assessment (LCA). The study demonstrates that waste-modified cement–soil composites intended for sustainable soil stabilization should be evaluated not only by strength parameters, but also by internal structure, defect distribution, and environmental impact.